This is an IRS con number. They leave a message that "This is your final notice that the IRS is about to level an lawsuit against you." These anal orifices prey on people, using the IRS scam to force people into sending money to them. The IRS does not work that way. If you are in trouble with back taxes, The real IRS will inform you my mail. they will never call you and threaten you.
However these guys are a lot of fun to play with. If you can get them to cuss you out and hang up on you, you win!